The present invention relates to electronic devices, and more particularly, to detecting faults in a power system.
Connecting two or more uninterruptible power supplies (UPS's) in a parallel redundant fashion can provide systems supporting higher and higher power availability. However, in order for a UPS to be able to operate in a redundant fashion with other UPS's, a robust “selective tripping” regime can be beneficial to provide system stability and functionality. As used herein, “selective tripping” refers to the set of hardware and/or associated software algorithms that can cause a failed UPS to trip off-line while the “good” parallel redundant unit(s) stay on-line. As will be appreciated by those skilled in the art, the redundant capability of a system can be limited by the effectiveness of its “selective tripping” regime.
UPS failures in a parallel redundant system include those that do not cause a disturbance on the critical output bus connected to the load, and those that do. The first kind of failure can be trivial since there is no disturbance on the critical bus. The good unit may not see the evidence of failure and may simply stay on-line doing its job. The failed unit may be simply off-line by virtue of its failure. However, the second kind of failure can be more problematic since all units may see the fault but any good units should stay on-line.
When a UPS fails in a way that the critical bus is disturbed, then it may be sourcing or sinking more current than can be absorbed or supplied by the other units that are on-line. Hence, all units may see the critical bus disturbance and all units may see excessive current. Yet, for selective tripping to work in this case, it is desirable that good units stay on-line and only the bad unit trip off-line. If a good unit trips off-line instead, then any benefit of redundancy for this type of failure can be lost.
Some previous parallel redundant power supply systems have used inter-unit signaling to facilitate selective tripping. For example, multiple uninterruptible power systems (UPS's) responsive to a three phase AC input voltage can be connected in a parallel redundant capacity configuration to provide a constant AC output voltage to a load. Each UPS can be connected to a DC voltage source (battery) in a conventional manner to provide constant power in the event of an AC power failure. Load sharing circuits can be associated with each UPS to facilitate inter-unit signaling and selective tripping using either a “difference from average” real time current signal on a “difference from average” bus, or a synchronizing signal provided on a synchronizing signal bus. Generally, if a fault is detected, the appropriate solid state or mechanical switch or relay can be thrown to remove the faulty UPS from the power circuit, thereby maintaining a constant AC output voltage on the load.
Using “difference from average” load sharing or common synchronization signal load sharing can be common practice. In the case of “difference from average” load sharing, for example, a real-time current signal can be used as an inter-unit signal amongst the parallel redundant UPSs. Unfortunately, the use of such inter-unit signaling can be vulnerable to interruption and hence should be redundant itself to provide a robust selective tripping regime. Since prior art systems can use inter-unit signaling for selective tripping, then there should be a redundant selective tripping scheme so that the inter-unit signaling itself does not become a single failure point of the otherwise redundant system.
Another common practice in the prior art can be to vectorially add the “difference from average” real-time current signal to the UPS's own voltage signal to create a biased voltage signal. This vector addition can be done in such a way that the “biased” voltage distortion on the failed unit is accentuated compared to the unbiased voltage while the same signal is less severe on the good unit(s). However, this method can require both signal comparison and logical processing to make the final determination of whether a unit is “good” or “bad”. While this method is usually adequate, the response may be slow due to the weakness of the difference. Also, this method can also rely on externally derived information such as the “difference from average” signal. Such additional signaling and the additional internal circuits used by the prior art parallel redundant power systems for identifying a faulty unit have had an adverse impact on reliability.
Selective tripping without inter-unit signaling has been previously addressed by monitoring each system's respective power output and keeping a running average of its own performance, as discussed in U.S. Pat. No. 5,745,355. Although power monitoring without inter-unit signaling is accomplished, the data processing and storage requirements associated with such methods can be memory and processor intensive. A more efficient technique for identifying and “selectively tripping” a faulty unit is desired.